The present invention relates generally to precision machines for mounting an optical element, and more particularly to a projection optical system in an exposure apparatus, etc. The present invention is suitable, for example, for a retainer that holds an optical element for a more precise imaging relationship in an exposure apparatus in projecting and exposing an image on an original sheet, such as a mask or reticle (these terms are used interchangeably in this application) onto an object, such as a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (LCD). The exposure apparatus is used to fabricate a semiconductor device, an image pick-up device (such as a CCD), and a thin film magnetic head.
The fabrication of a device using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a circuit pattern formed on a mask onto a wafer and the like, thereby transferring the circuit pattern. The projection optical system enables diffracted beams from the circuit pattern to interfere on the wafer and the like, so as to form an image.
The devices to be mounted on electronic apparatuses should be highly integrated to meet recent demands for miniaturization and low profile of electronic apparatuses, and finer circuit patterns to be transferred or higher resolution have been demanded increasingly. A short wavelength of a light source and an increased numerical aperture (“NA”) in a projection optical system are effective to the high resolution as well as a reduced aberration in the projection optical system.
An optical element, such as a lens and a mirror, when deforming in an projection optical system causes aberration because an optical path refracts before and after the deformation and light that is supposed to form an image at one point does not converge on one point. The aberration causes a positional offset and short-circuits a circuit pattern on a wafer. On the other hand, a wider pattern size to prevent short-circuiting is contradictory to a fine process.
Therefore, a projection optical system with small aberration should hold its optical element(s) without changing a shape and a position relative to the optical axis of the optical element in the projection optical system so as to maximize the original optical performance of the optical element. In particular, the projection lens tends to have a larger caliber and a larger lens capacity due to the recent high NA in the projection optical system, and easily deforms by its own weight. In addition, diffraction optical elements, which have been eagerly studied recently, also tend to deform due to its thinness.
Accordingly, Japanese Patent Application, Publication No. 2001-284226 proposes, as shown in FIG. 11, a retainer 1000 that fixes a lens 1300 using three or more (e.g., thirty in the embodiment) lens support points 1200 formed on a cell 1100, and springs 1400 for pressing the lens 1300 from a side opposite to the lens support points 1200. A compression force is designed or calculated to the extent that a lens of a low breaking strength, such as a lens made of calcium fluoride (CaF2), does not get damaged. Here, FIG. 11 is an exploded perspective view of the exemplary conventional retainer 1000.
Similarly, Japanese Patent Application,
Publication No. 2001-74991 (corresponding to U.S. Pat. No. 6,239,924) proposes, as shown in FIG. 12, a retainer 2000 that fixes a lens installed onto seats on three flex mount parts 2200 that are radially arranged at regular intervals on an inner circumference on a cell 2100, and a spring member with small rigidity in a non-optical axis direction for applying a compression force to the lens from the top of the lens according to the lens shape. Soft mount parts 2300 dispersedly support the lens among the flex mount parts 2200 so as to minimize the gravity deflection and so as not to excessively restrain the positioning of the lens. Here, FIG. 12 is a schematic top transparent view showing the exemplary conventional retainer 2000.
However, Japanese Patent Application, Publication No. 2001-284226 does not consider the flatness of the lens support points 1200 and lens deformation. For example, when the heights of the lens support points 1200 deviate, only three points among them support points 1200 even when there are thirty or more lens. Therefore, the portion except the above three support points does not contact the lens but receives forces from the springs 1400, causing the lens to deform. Thus, this reference has a difficulty in realizing a projection optical system with little aberration due to such a lens's deformation.
On the other hand, Japanese Patent Application, Publication No. 2001-74991 has a careful structure to a lens deformation or distortion, but requires forces opposite to those applied by the flex mount parts at three flex mount parts 2200 so as to apply a compression force to prevent the lateral offset of the lens. This causes the lens to incur a large load from the spring members of the flex mount parts 2200. This causes the large birefringence and possibly breaks the lens, and has a difficulty in realization.